1 . Field of the Invention
The present invention relates to an electronic-parts mounting board, and an electronic-parts mounting board frame with a plurality of electronic-parts mounting boards assembled thereinto.
2. Discussion of the Related Art
Various types of electronic-parts mounting boards have been proposed. One of those known electronic-parts mounting boards is an electronic-parts package, or a semiconductor device. In the package, after an electronic part is mounted on the board, the board is entirely covered for sealing purposes with a so-called resin mold, while the outer leads of the semiconductor device protrude outwardly from the resin mold.
An electronic-parts mounting board sealed with a resin mold is disclosed in Japanese Patent Laid-Open Publication No. Sho. 59-98545, entitled "Pellet Mounting Board," which is for a semiconductor device. The publication claims a part or "pellet" mounting board "wherein a pellet is mounted on a pellet mounting board with a conductive layer, bonding pads of said pellet are electrically connected to said conductive layer by a wire bonding method, and said conductive layer of said pellet mounting board is bonded to a lead frame." The disclosed pellet mounting board suffers from the following problems, however:
1) Solder is used for bonding the pellet mounting board (referred to as a board) to the conductive layer. Each portion (pad) subjected to the soldering lies on an insulating board. Accordingly, the pad tends to peel off the insulating board when it is heated for soldering.
2) The insulating board is located between the pads. Solder mistakenly left on the insulating board possibly may cause an electrical short between the pads.
3) The thermal expansion coefficient of the board is different from that of the lead frame. When the pellet (electronic part) is heated, the lead frame tends to peel off the board. One possible cause of the peel-off is warping of the lead frame.
4) The major surfaces of the insulating board do not fully utilize the space available to form the conductive circuits.
To cope with the above problems, an electronic-parts mounting board has been proposed which is constructed such that an insulating board is applied to both sides of the lead frame, to form a one piece construction, and the conductive circuits on the insulating board are electrically connected to the leads through the through-holes.
To be more specific, the proposed mounting board uses lead frames each having leads, connected by tie bars, that are constructed such that inner leads are electrically connected to the conductive circuits through the through-holes, and outer leads protrude from both the insulating board and the resin mold and are physically continuous.
The electronic-parts mounting board thus constructed still involves unsolved problems, however, as follows:
5) In recent usage of the electronic-parts mounting board, a plurality of electronic-parts mounting boards are assembled into a frame, in order to routinely and efficiently mount electronic parts on the board and to seal the structure with a resin mold. To assemble the insulating board based electronic-parts mounting boards into the lead frame, a single metal plate is punched to form leads in the locations where a plurality of electronic-parts mounting boards are to be formed. Insulating boards, separately formed, are coupled with the leads.
6) In the electronic-parts mounting boards thus assembled into one frame, if a part of the lead frame is defective, the whole lead frame must be treated as being defective. Accordingly, the remaining non-defective parts of the lead frame are wasted. The portions of the lead frame to be used for the electronic-parts mounting boards require especially delicate working for their formation. The cost to form those portions is higher than that to form other portions. Therefore, replacement of only those portions to be used for the electronic-parts mounting boards with non-defective portions would be very advantageous.
7) Since a plurality of electronic-parts mounting boards are assembled into a single lead frame, the whole lead frame must be individually designed and manufactured so as to have a shape in conformity with the shape of the electronic-parts mounting boards. In this respect, the versatility of the lead frame in usage is poor.
8) The electronic-parts mounting board must be subjected to electrical checks or tests in the stages of manufacturing the board. Simplification of the process of subjecting the board to electrical testing would be highly advantageous.
Studies made by the inventors to solve the above problems and to reduce the disadvantages of the electronic-parts mounting board have resulted in the present invention.